A. Technical Field
Fabrication of small circuits and circuit elements, e.g., large scale integrated circuits (LSI) is realized through one or more pattern delineation steps. In accordance with prevalent practice at this time, use is made of discrete masks which, when finally processed, consist of apertured chromium patterns supported on glass substrates. Typically a set of six or more such masks are required for semiconductor circuit fabrication. They are utilized sequentially for replicating patterns in sensitive supported (resist) material on the semiconductor, following which the replicated pattern is utilized to define areas to be etched, plated, implanted, or otherwise processed. There is a growing technology which involves electron beam delineation to produce such masks with design rules of a few micrometers or less. Replication is generally accomplished with near u.v. light.
The expectation that economic and other considerations may lead to smaller design rules has focused attention on inherent limitations in presently used mask technology. Standing waves, interference and other limitations relating to wavelength have led to studies directed to the use of effectively shorter wavelength replicating radiation such as X-ray, electron flood, and short wavelength u.v. Anticipated yield loss due to registration difficulty with diminishing design rules is leading to evolution of a "maskless" technology known as "direct processing." In this technology, the primary pattern delineation is in resist layers adherent to the device at each stage in fabrication. All such fine-line programs are dependent upon suitable resists.
B. History
A variety of extremely sophisticated negative resist materials have been developed to meet circuit fabrication needs. A resist now in widespread commercial use is based on an addition polymer of glycidyl methacrylate (GMA) and ethyl acrylate (EA) (see, 12, J. Vac. Sci. Technol., 1957 (1975). The material is excellent for its intended purpose of mask fabrication based on pattern design rules of a small number of microns. It is of limited utility for maskless fabrication (direct processing), since it degrades at a significant rate in certain processing environments.
Improved stability to dry processing characterizes a family of resists which are chemically related to GMA-EA. Again, the primary radiation-responsive mechanism, cross linking via epoxy moieties, is sufficiently sensitive to permit use of usual energy sources; e.g., tungsten or thorated tungsten electron sources in commercial raster scan or vector scan 1:1 mask makers. Inclusion of aryl moieties largely in lieu of EA results in a significant increase in stability attributed to the inherent resonant nature of the substituent. Loss in lithographic sensitivity is, in part, retrieved through halogenation of the aryl grouping, e.g., GMA-chlorostyrene sometimes denoted as GMC.
An outgrowth of the GMC effort resulted in the finding that polychlorostyrene itself has promising resist properties. Borderline sensitivity (while sufficient for many purposes is only barely adequate for use on some existing primary pattern e-beam generators) is to a large extent offset by dry processing stability.
It is the experience of workers in the field, that the best negative resists have less resolving power than the best positive resists. Resolution limitations observable as ragged interfaces at feature edges are generally attributed to backscattered energy. This limitation is particularly significant in electron beam lithography where design rules are likely small and where backscattered electrons are sufficient to cause some crosslinking of "unexposed material." It follows that a significant characteristic of a negative resist--particularly a negative resist designed for fine-line lithography--is the breadth of the incident radiation dose which results in insolubilization, e.g., crosslinking. This characteristic in turn is dependent upon molecular weight distribution with the very important parameter of breadth of radiation dose required for crosslinking decreasing as molecular weight distribution approaches the theoretical unity level. This relationship is readily understood on the basis that requisite insolubilization on the average results from but one crosslink per molecule so that effective sensitivity varies with polymer weight (increases with increasing polymer weight). It is therefore well acknowledged that narrow molecular weight distribution is a desirable feature--one of increasing importance with decreasing design rules.
Resists of the nature described above, generally produced by free radical polymerization, are characterized by molecular weight distributions ("MWD") numerically greater than 2, as usually defined. The parameter here is the ratio: weight average molecular weight/number average molecular weight (M.sub.w /M.sub.n). This parameter as so defined is in prevalent use (see for example L. H. Peebles, Jr., Molecular Weight Distributions in Polymers, Interscience, (1971)).